Global warming and climate change are presently receiving significant scientific, business, regulatory, political, and media attention. According to increasing numbers of independent scientific reports, greenhouse gases impact the ozone layer and the complex atmospheric processes that re-radiate thermal energy into space, which in turn leads to global warming on Earth. Warmer temperatures in turn affect the entire ecosystem via numerous complex interactions that are not always well understood. Greenhouse gases include carbon dioxide, but also include other gases such as methane, which is about 23 times more potent than carbon dioxide as a greenhouse gas, and nitrous oxide, which is over 300 times more potent than carbon dioxide as a greenhouse gas.
In addition to the foregoing greenhouse gas concerns, there are significant concerns about the rate at which oil reserves are being depleted and that the United States imports over 60% of the crude oil it consumes from a few unstable regions of the globe. Accordingly, there is an increasing focus on finding alternative sources of energy, including clean, renewable, less expensive, and domestic energy sources. These sources include municipal solid waste, food processing wastes, animal wastes, restaurant wastes, agricultural wastes, and waste water treatment plant sludge. These sources also include coal seam methane, coal mine gas, and stranded well gas.
Many of the foregoing organic waste streams can be converted to biogas via anaerobic bacteria to produce mixtures of methane. Examples include covered landfills where the landfill gas contains approximately 48% methane, 38% carbon dioxide, 12% nitrogen and oxygen, water vapor, and small amounts of numerous other compounds. Biogas from anaerobic digestion of organic waste streams consists of approximately 65% methane, 33% carbon dioxide, water vapor and small amounts of other compounds. Coal mine gas contains approximately 64% methane, 32% nitrogen, 3% carbon dioxide, water, and small amounts of other compounds. Stranded well gas has a wide range of compositions depending on the location but typically contains approximately 80% methane, 13% nitrogen, several percent ethane and propane, plus water and 2% carbon dioxide. These stranded or waste sources of methane are widely geographically distributed rather than in large, localized sources like a large gas field. With enhanced technology these distributed methane sources are being converted to compressed natural gas (CNG) or liquid natural gas (LNG) and used as low emissions vehicle fuel for light and heavy duty vehicles.
The processes associated with producing both LNG and CNG include purifying the incoming methane gas stream to remove, among other constituents such as those that freeze out in or otherwise degrade LNG process equipment, constituents that contribute to an increase in emitted greenhouse gases. For example, the carbon dioxide present in the methane mixtures from most of the distributed waste or stranded sources must be efficiently removed to a concentration of about 100 parts per million to avoid freezing out in LNG. The small, distributed nature of many biomass waste streams or stranded gas wells or landfills makes the capital and operating costs associated with a waste-to-energy plant a key component in the price of the LNG or CNG vehicle fuel. Accordingly, there is a need for better purifier and liquefier technology that reduces capital and operating costs and results in a more competitively priced methane fuel.